The present invention relates to a polyethylene moulding compound having a multimodal molecular weight distribution and to a method for the production of this moulding compound in the presence of a catalytic system comprising a Ziegler catalyst and co-catalyst via a multistep reaction sequence consisting of successive liquid-phase polymerizations, and to hollow articles produced from the moulding compound by extrusion blow moulding.
Polyethylene is widely used for the production of mouldings and containers since it is a material having a low inherent weight which nevertheless has particularly high mechanical strength, high corrosion resistance to moisture and water in combination with atmospheric oxygen and absolutely reliable long-term resistance and since polyethylene has good chemical resistance and in particular can easily be processed for bottles, canisters and fuel tanks in motor vehicles.
EP-A-603,935 has already described a moulding compound based on polyethylene which has a bimodal molecular weight distribution and which is also suitable, inter alia, for the production of pipes.
A raw material having an even broader molecular weight distribution is described in U.S. Pat. No. 5,338,589 and is prepared using a highly active catalyst disclosed in WO 91/18934 in which magnesium alkoxide is employed in the form of a gelatinous suspension. Surprisingly, it has been found that the use of this material in mouldings, in particular in pipes, facilitates a simultaneous improvement in the properties of stiffness and creep tendency, which are usually contradictory in partially crystalline thermoplastics, on the one hand, and stress cracking resistance and toughness on the other hand.
The known bimodal products are distinguished, in particular, by good processing properties at the same time as an outstanding stress cracking/stiffness ratio. This combination of properties is of particular importance in the production of hollow articles from plastics, such as bottles, canisters and fuel tanks in motor vehicles. In addition to this property combination, however, the production of plastic hollow articles requires the highest possible swelling rate of the plastic melt, since the swelling rate is directly responsible for enabling the optimum setting of wall thickness control, the formation of the weld line and the weldability in industrial production in extrusion blow moulding.
It is known that having high swelling rates can be produced well using so-called Phillips catalysts, i.e. polymerization catalysts based on chromium compounds. However, the plastics produced in this way have an unfavourable stress cracking/stiffness ratio compared with the known plastics having a bimodal molecular weight distribution.
EP-A-0 797 599 discloses a process which even gives a polyethylene having a trimodal molecular weight distribution in successive gas-phase and liquid-phase polymerizations. Although this polyethylene is already very highly suitable for the production of hollow articles in extrusion blow moulding plants, it is, however, still in need of further improvement in its processing behaviour owing to the plastic melt swelling rate, which is still too low.
The object of the present invention was the development of a polyethylene moulding compound by means of which an even better ratio of stiffness to stress cracking resistance compared with all known materials can be achieved and which, in addition, has a high swelling rate of its melt, which, in the production of hollow articles by the extrusion blow moulding process, not only enables optimum wall thickness control, but at the same time also facilitates excellent weld line formation and wall thickness distribution.
This object is achieved by a moulding compound of the generic type mentioned at the outset, whose characterizing features are to be regarded as being that it comprises from 30 to 60% by weight of a low-molecular-weight ethylene homopolymer A, from 65 to 30% by weight of a high-molecular-weight copolymer B comprising ethylene and another olefin having from 4 to 10 carbon atoms, and from 1 to 30% by weight of an ultrahigh-molecular-weight ethylene homopolymer or copolymer C, where all percentages are based on the total weight of the moulding compound.
The invention furthermore also relates to a method for the production of this moulding compound in cascaded suspension polymerization, and to hollow articles made from this moulding compound with very excellent mechanical strength properties.
The polyethylene moulding compound according to the invention has a density in the range xe2x89xa70.940 g/cm3 at a temperature of 23xc2x0 C. and has a broad trimodal molecular weight distribution. The high-molecular-weight copolymer B comprises small proportions of up to 5% by weight of further olefin monomer units having from 4 to 10 carbon atoms. Examples of comonomers of this type are 1-butene, 1-pentene, 1-hexene, 1-octene or 4-methyl-1-pentene. The ultrahigh-molecular-weight ethylene homopolymer or copolymer C may optionally also comprise an amount of from 0 to 10% by weight of one or more of the above-mentioned comonomers.
The moulding compound according to the invention furthermore has a melt flow index, in accordance with ISO 1133, expressed as MFI190/5, in the range from 0.01 to 10 dg/min and a viscosity number VNtot, measured in accordance with ISO/R 1191 in decalin at a temperature of 135xc2x0 C., in the range from 190 to 700 cm3/g, preferably from 250 to 500 cm3/g.
The trimodality can be described as a measure of the position of the centres of the three individual molecular weight distributions with the aid of the viscosity numbers VN in accordance with ISO/R 1191 of the polymers formed in the successive polymerization steps. The following band widths of the polymers formed in the individual reaction steps should be taken into account here:
The viscosity number VN1 measured on the polymer after the first polymerization step is identical with the viscosity number VNA of the low-molecular-weight polyethylene A and is in accordance with the invention in the range from 40 to 180 cm3/g.
VNB of the relatively high-molecular-weight polyethylene B formed in the second polymerization step can be calculated from the following mathematical formula:       VN    B    =                    VN        2            -                        w          1                ·                  VN          1                            1      -              w        1            
where w1 represents the proportion by weight of the low-molecular-weight polyethylene formed in the first step, measured in % by weight, based on the total weight of the polyethylene having a bimodal molecular weight distribution formed in the first two steps, and VN2 represents the viscosity number measured on the polymer after the second polymerization step. The value calculated for VNB is normally in the range from 150 to 800 cm3/g.
VNC for the ultrahigh-molecular-weight homopolymer or copolymer C formed in the third polymerization step is calculated from the following mathematical formula:       VN    C    =                    VN        3            -                        w          2                ·                  VN          2                            1      -              w        2            
where w2 represents the proportion by weight of the polyethylene having a bimodal molecular weight distribution formed in the first two steps, measured in % by weight, based on the total weight of the polyethylene having a trimodal molecular weight distribution formed in all three steps, and VN3 represents the viscosity number which is measured on the polymer after the third polymerization step and is identical with the VNtot already mentioned above. The value calculated for VNC is in accordance with the invention in the range from 900 to 3000 cm3/g.
The polyethylene is obtained by polymerization of the monomers in suspension or at temperatures in the range from 20 to 120xc2x0 C., a pressure in the range from 2 to 60 bar and in the presence of a highly active Ziegler catalyst composed of a transition-metal compound and an organoaluminium compound. The polymerization is carried out in three steps, i.e. in three successive steps, with the molecular weight in each case being regulated with the aid of metered-in hydrogen.
The polymerization catalyst""s long-term activity, which is necessary for the cascaded procedure described above, is ensured by a specially developed Ziegler catalyst. A measure of the suitability of this catalyst is its extremely high hydrogen responsiveness and its high activity, which remains constant over a long period of from 1 to 8 hours. Specific examples of a catalyst which is suitable in this manner are the products cited in EP-A-0 532 551, EP-A-0 068 257 and EP-A-0 401 776 of the reaction of magnesium alkoxides with transition-metal compounds of titanium, zirconium or vanadium and an organometallic compound of a metal from groups I, II or III of the Periodic Table of the Elements.
Besides the polyethylene, the polyethylene moulding compound according to the invention may also comprise further additives. Additives of this type are, for example, heat stabilizers, antioxidants, UV absorbers, light stabilizers, metal deactivators, peroxide-destroying compounds, basic costabilizers in amounts of from 0 to 10% by weight, preferably from 0 to 5% by weight, but also fillers, reinforcing agents, plasticizers, lubricants, emulsifiers, pigments, optical brighteners, flame retardants, antistatics, blowing agents or combinations thereof in total amounts of from 0 to 50% by weight, based on the total weight of the mixture.
The moulding compound according to the invention is particularly suitable for the production of hollow articles, such as fuel canisters, chemical-resistant containers, canisters, drums and bottles, by firstly plasticating the polyethylene moulding compound in an extruder at temperatures in the range from 200 to 250xc2x0 C., and then extruding the compound through a die into a blow mould, and cooling it therein.
For conversion into hollow articles, use can be made of both conventional single-screw extruders having a smooth feed zone and high-performance extruders having a finely grooved barrel and forced conveying feed section. The screws are typically designed as decompression screws with a length of from 25 to 30 D (D=diameter). The decompression screws have a discharge zone in which temperature differences in the melt are compensated and in which the relaxation stresses formed due to shearing are intended to be dissipated.